1. Field of the Invention
The present invention relates to a coil of tapered wire for forming a coil spring, formed by coiling a tapered wire continuously varying in section in substantially concentric loops
2. Description of the Prior Art
Generally, the conventional coil spring is formed by coiling a wire having a fixed diameter through a cold working as illustrated in FIGS. 3(1) and 3(2). A coil 21 formed by coiling a wire 25 having a fixed section substantially in concentric loops is mounted on a turn table 22. The wire 25 is pulled out from the coil 21 through a straightening unit 26 by the feed rollers 24 of a spring forming machine 23. The straightening unit 26 has an alternate arrangement of three fixed straightener rollers 26a and two movable straightener rollers 26b. The wire 25 is straightened by the straightening unit 26 so as to extend in a straight line in an unrestrained condition. The straightened wire 25a is subjected to the coiling action of coiling members 27 for cold coiling. The coiling members 27 need not necessarily be rolling members as shown in FIGS. 3(1) and 3(2), but may instead be fixed dies. The degree of straightening action of the straightening unit 26 is regulated by varying the disposition of the two movable rollers 26b relative to the three fixed straightener rollers 26a. The respective number of the straightener rollers 26a and 26b need not necessarily be three and two, but may be optional numbers.
In recent years, a coil spring continuously varying in section as shown in FIG. 4, namely, a tapered wire coil spring 1 having nonlinear spring characteristics has progressively become used instead of a coil spring having a fixed section, to improve the riding comfort and to reduce the weight of automobiles and railway vehicles.
Such a tapered wire coil spring 1 is formed by coiling a tapered wire 3 having a thick section 4, thin end sections 5 and 5', and tapered sections 6 connecting the thin end sections 5 and 5' to opposite ends of the thick section 4, respectively, as shown in FIG. 5. The diameter d.sub.2 of the thin end section 5 and the diameter d.sub.3 can be either d.sub.2 =d.sub.3 or d.sub.2 .noteq.d.sub.3. In the following description, the thin end sections 5 and 5' will be denoted inclusively by a reference numeral 5.
Generally, the tapered wire 3 is manufactured by using a swaging machine capable of high-speed forging or a peeling machine capable of high-speed peeling. The applicant of the present invention proposed previously a highly productive tapered wire manufacturing process in Japanese Patent Provisional Publication Nos. 60-56416 and 60-56417.
In this tapered wire manufacturing process, a metallic material is fed at a fixed feeding speed into a heating furnace for softening, then the softened metallic wire is cooled, and the cooled metallic wire is then drawn at a regularly variable drawing speed so that the sectional area of the softened and cooled metallic wire is varied continuously to form a tapered wire 3. The tapered wire 3 is wound in a coil before being supplied to a spring forming machine 23.
However, problems arise in winding the tapered wire 3 in a coil. Since the thin end sections 5 are smaller than the thick section 4 in section, the thin end sections 5 are subject to plastic strain, so that the thin end sections 5 are liable to be bent sharply as shown in FIGS. 6(1) and 6(2) when the tapered wire 3 is wound in a coil. Therefore, the tapered wire 3 thus wound in a coil assumes as unrestrained tapered wire 3a as shown in FIG. 7 in an unrestrained state. That is, the unrestrained tapered wire 3a has thin end sections 5 greater in plastic bending characteristic than the thick section 4, namely, R.sub.1 &gt;R.sub.2, where R.sub.1 is the radius of curvature of the thick section 4 and R.sub.2 is the radius of curvature of the thin end sections 5. A coiled wire having a uniform section is uniform in plastic bending characteristic in an unrestrained state. The degree of plastic bending is dependent on the limit of elasticity and diameter of the wire, the diameter of the winding drum and the magnitude of back tension irrespective of the position on the wire.
To form a coil of the tapered wire 3a having plastic bends as shown in FIG. 7, the tapered wire 3a needs to be straightened by the straightening unit 26 in a manner as described with reference to FIGS. 3(1) and 3(2) before coiling. However, the horizontal distance, the distance in a direction perpendicular to the sheet in FIG. 3(2), between the fixed straightener rollers 26a and the movable straightener rollers 26b is unavoidably fixed and hence the straightening action of the straightening unit 26 on the thick section 4 is more intensive than that on the thin section 5. Therefore, the thin section 5 cannot be straightened satisfactorily.
FIGS. 8(1) and 8(2) show modes of straightening operation of the straightening unit 26 with the fixed straightener rollers 26a and the movable straightener rollers 26b set in a fixed straightening arrangement, namely, with the center distance C.sub.1 between the fixed straightener rollers 26a and the movable straightener rollers 26b for straightening a thicker wire 28a (FIG. 8(1)) and the center distance C.sub.2 between the fixed straightener rollers 26a and the movable straightener rollers 26b for straightening a thinner wire 28b (FIG. 8(2)) equal to each other. As is obvious from FIGS. 8(1) and 8(2), such an arrangement of the fixed straightener rollers 26a and the movable straightener rollers 26b is effective for straightening the thicker wire 28a, however, the same is unable to straighten the thinner wire 28b satisfactorily.
When a tapered wire 3 thus unsatisfactorily straightened on a straightening machine with the straightening rollers set in a fixed straightening arrangement is subjected to a coil spring forming process, the plastic bending of the thin sections of the tapered wire 3 remains almost the same as in forming a tapered wire coil spring 1' having an irregular form as shown in FIG. 9.
It is possible to straighten such as tapered wire satisfactorily by automatically regulating the straightening arrangement of the straightener rollers according to the variable diameter of the tapered wire and the degree of bends in the tapered wire. However, such a straightening process needs a sophisticated control system including a detecting unit for detecting the continuously variable diameter and bends of the tapered wire and a control unit capable of continuously controlling the straightening arrangement of the straightener rollers on the basis of information provided by the detecting unit. Accordingly, such a straightening process, in practice, is not readily feasible.
It is also possible to form a uniform tapered wire coil spring by continuously controlling the coil forming rollers so as to move radially of the tapered wire coil spring in winding a tapered wire having plastic bending in the thin sections 5. However, this process, similarly to the former process, needs a sophisticated control system and hence is not readily feasible.
Therefore, according to the technical status quo, straight tapered wire 3 having a fixed length is subjected to a coil spring forming process, and hence coil springs cannot be formed, thus continuously deteriorating the efficiency of the coil spring manufacturing process.